Fluid transfer devices (e.g., pipette mechanisms and pumps) are used to aspirate, dispense and transfer small volumes of fluid in many applications. The devices may range from simple glass tubes to more elaborate mechanical displacement devices. In either case, the devices operate by displacing fluid and a seal is used to hold the displaced fluid, which facilitates the liquid transfer. Traditional devices use displacement pistons with mechanical seals, such as lip seals or o-rings, to prevent air from entering the displacement chamber. These seals can be run dry, and wear eventually causes the seal to leak and degrades accuracy of the device.
Such devices may use a linear actuator to provide linear motion to the displacement piston. Couplings and other structures may be used to couple the linear actuator to the displacement piston. Misalignment of the actuator to the piston may result in premature seal degradation and may adversely affect the accuracy of the device during fluid transfer. Magnetic couplers may be used to prevent axial and/or angular misalignment.
Although magnetic couplers prevent axial and/or angular misalignment, the attractive forces of the magnets sometimes result in side loads on the piston. For example, the magnets may tend to center themselves and a substantial lateral force may be required to overcome the attractive forces of the magnets, resulting in a somewhat rigid magnetic coupling. This rigidity and magnetic resistance may translate into side loading on the piston against the wall, which may adversely affect performance of a displacement apparatus using the magnetic coupling system, particularly in precision dispensing applications.